Method and circuit arrangement for the transmission of message cells during the course of virtual connections having different priorities

ABSTRACT

Message cells that are allocated to virtual connections having different priorities are transmitted jointly over at least one transmission line of an ATM system working according to an asynchronous transfer mode. While being transmitted via the respective transmission lines, these message cells pass through a buffer memory individually allocated thereto. Given a buffer memory exhibiting a specific filling degree, one or more message cells from message cells of a selected virtual connection of low priority, which are, stored at the moment in the buffer memory, are discarded with the arrival of a message cell on the respective transmission line belonging to a virtual connection of relatively high priority, and dependent on the number of message cells stored for this virtual connection having low priority.

BACKGROUND OF THE INVENTION

The present invention is directed to a method and to a circuitarrangement for the transmission joint of message cells, which areallocated to virtual connections having different priorities, via atleast one transmission line of an ATM system working according to anasynchronous transfer mode, whereby, during transmission via therespective transmission line, the message cells pass through a buffermemory individually allocated thereto.

In such an ATM system, a number of virtual connections, for whichdifferent demands are made with respect to the cell loss probabilities(different priorities), can be simultaneously established on atransmission line. For example, a cell loss probability of 10⁻⁶ can bedemanded for one of the virtual connections but no cell loss probabilitymay be demanded for another virtual connection.

European Patent Application No. 0 641 099 discloses a method and anapparatus for the intermediate storage of data. In accord therewith, adata packet stored in a low-priority wait list is erased when anincoming data packet is allocated to a wait list with higher priorityand the total length of the wait lists has exceeded a certain length.However, an optimum loading of the wait lists or of the buffer memoriesforming these is not possible with such a procedure.

European Patent Application No. 0 526 104 likewise discloses a methodand an apparatus for the intermediate storage of data packets. A datapacket an the lowest-priority wait list is discarded when the totallength of the wait lists has exceeded a certain length. The newly addeddata packet is then written to the empty location in the wait list.However, an optimum loading of the wait lists or of the buffer memoriesforming these is likewise not possible with such a procedure.

SUMMARY OF THE INVENTION

It is an object of the present invention is to provide a method and acircuit arrangement for efficently loading the transmission lines of anATM system and the buffer memories respectively allocated to these.

This object is achieved in accordance with the invention in a method ofthe species initially cited. Message cells are transmitted via atransmission line through a buffer memory individually allocated to thetransmission line and having a specific message cell capacity. A messagecell belonging to a high-priority virtual connection on the transmissionline is received. A wait list allocated to a low-priority virtualconnection of message cells stored in the buffer memory is selected uponreceiving the message cell belonging to a high-priority virtualconnection and upon the buffer memory achieving the specific messagecell capacity. The selection depends on the number of message cellsbeing stored in the buffer memory for the low-priority virtualconnection. The message cells of said selected wait list are thendiscarded.

The invention thereby yields the advantage that, viewed overall, fewerconnections are damaged on average by cell losses. Above all, in datatraffic wherein data packets of a virtual connection are initiallysegmented before a transmission over the transmission line coming intoconsideration and the individual segments are subsequently transmittedin a respective message cell, discarding individual message cells isinefficient. In order to avoid data errors, namely, the receiverinvolved in a virtual connection must discard an entire data packagedamaged by cell loss. Moreover, the number of data packets transmittederror-free on average, i.e. data packets that suffer no cell loss, isincreased.

Over and above this, the present invention avoids a random discard ofindividual message cells as a result whereof there would be highprobability that many virtual connections would be disturbed.

Advantageous developments of the method according to the presentinvention derive from further embodiments of the invention

In further embodiments, all or all except a first of the message cellsof a selected low-priority virtual connection are discarded.

In a further embodiment, a virtual connection having a greatest numberof message cells is selected.

In a further embodiment, the selection depends on the priorities of thevirtual connections and the number of message cells stored for theindividual virtual connections.

The advantage is comprised in a further increase in the efficientloading of the transmission lines and the buffer memories allocated tothem. What is achieved, for example, by discarding all except the firstmessage cells of the selected low-priority virtual connection thecontinuity of the connecting paths can be checked adequately in an ATMsystem.

Over and above this, present invention makes it possible for thosevirtual connections for which many message cells are being buffered atthe moment and that, thus, contribute greater than others to anoverload, are preferably utilized for discarding message cells. Forexample, these are virtual connections that do not adhere to definedconnection parameters, for example that exceed the cell rate defined asa maximum for the respective virtual connection.

The above-stated object is achieved in a circuit arrangement operated inaccordance with the inventive method. The advantages thereby comprisedin what is a low circuit-oriented outlay overall in order to efficientlyload the individual transmission lines and the buffer memories allocatedto them.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit diagram of a circuit arrangement constructedand operated in accordance with the invention.

FIG. 2 is a schematic circuit diagram for the formation of a logicalweight list.

FIG. 3 is a schematic circuit diagram for discarding the cells of alogical weight list.

DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT

FIG. 1 schematically shows line equipment LE that is inserted betweentwo transmission line sections L1 and L2 of an ATM system workingaccording to an asynchronous transfer mode. Only those circuit elementsof the line equipment LE that are required for an understanding of thepresent invention are thereby shown. Over and above this, this lineequipment LE stands for further line equipment inserted intotransmission line sections of the ATM system.

The general ATM principle is not discussed in detail below, since thisis generally known in the art.

Message cells (“cells”) occur on the transmission line section L1, thesehaving an external cell header (“header”) available to them in a knownway in addition to an information part (“user part”). The particulars insuch an external cell header thereby include affiliation to a specificvirtual connection. A virtual connection can be a matter either of avirtual channel connection (individual connection) or of a virtual pathconnection (bundle of a number of individual connections). A virtualchannel connection thereby has a virtual channel number VCI (“virtualchannel identifier”) allocated to it but a virtual path connection has avirtual path number VPI (“virtual path identifier”) allocated to it inthe external cell header of the respective message cell. In a case of avirtual path connection, a virtual channel number VCI is also indicatedin the external cell header in order to be able to identify theindividual virtual channel connections conducted within the virtual pathconnection.

The input of the line equipment LE (FIG. 1) is formed by a memorycontroller CONV. This places an internal cell header (“header”) in frontof every message cell appearing on the transmission line section L1 inorder to be able to forward the respective message cell within the ATMsystem. This internal cell header is formed according to the criterionof the content of the external cell header respectively contained in themessage cells. Among other things, one of m weight list identifiers QID(“Q identifications”) is statistically allocated according to thecriterion of the VCI or, VPI/VCI contained in the respective externalcell header. The respective weight list identifier QID effects anaddress reduction compared to the allocated VCI or, VPI/VCI and isaccepted into the pertaining internal cell header.

The message cells expanded in this way by an internal cell header aresuccessively supplied to a demultiplex means DEMUX (FIG. 1), that isconnected via m demultiplex outputs to a buffer memory PS. Thedemultiplex outputs are individual allocated to the weight listidentifiers QID.

The buffer memory PS comprises a number of memory locations from which amaximum m logical weight lists can be formed, as shall be explained ingreater detail later. These logical weight lists, which are referencedQ1 through Qm in FIG. 1, are individually selected by the demultiplexmeans DEMUX with the assistance of the weight list identifiers QIDcontained in the received message cells. In response to such a selectionof a logical weight list, the message cell provided with the weight listidentifier QID coming into consideration at the moment is transferredthereinto. The individual logical weight lists Q1-Qm therebyrespectively form an FIFO memory (“first-in-first-out” memory) that cansimultaneously intermediately store a number of message cells.

At the output side, the logical weight lists (Q1 through Qm) areselected in a defined sequence by an operating means SC (“scheduler”)within the framework of a predetermined cell scheduling. In such a readcycle, a message cell is thereby taken from each logical weight listQ1-Qm and forwarded via the transmission line section L2.

The logical weight lists Q1-Qm can exhibit different degrees of fillingdependent on the traffic volume on the pertaining transmission linesection L1 Given a determined filling capacity of the buffer memory PS(FIG. 1), it is provided according to the present invention that, giventhe arrival of a message cell that is allocated to a virtual connection(individual connection or path connection) having relatively highpriority (high demand made of the cell loss probability), a definednumber of message cells of a selected, virtual connection of a lowerpriority is discarded, i.e. this number of message cells is deleted inthe logical weight list Q1-Qm of the buffer memory PS allocated to thisvirtual connection at the moment. The number can thereby be dependent,for example, on the length of the logical weight list Q1-Qm. Thus, forexample, all message cells stored in the pertaining logical weight listQ1-Qm can also be discarded, i.e. deleted.

Alternatively thereto, one can also proceed such that all message cellsstored in the pertaining logical weight list Q1-Qm at the moment exceptthe first thereof are discarded. What is thus achieved is that thecontinuity at the individual connection paths can be checkedsufficiently often in the ATM system, which is only partially shown inFIG. 1.

Thresholds for the individual logical weight lists Q1-Qm can be definedfor the above-described discarding of message cells, whereby, when alogical weight list Q1-Qm upwardly exceeds such a threshold, the virtualconnection allocated thereto is preferably utilized for the discarding.

Given the presence of a number of logical weight lists Q1-Qm, uponconsideration of discarding of message cells, the selection can ensueeither in a defined sequence or according to a random principle. Forexample, the priority of the virtual connections allocated to the weightlists Q1-Qm can be utilized as selection criterion for these logicalweight lists Q1-Qm. Alternatively or additionally, the momentary lengthof the individual logical weight lists Q1-Qm can serve as furthercriterion.

With these last-cited alternatives, it is possible that virtualconnections that contribute greater than other virtual connections to anoverload are taken into consideration with priority for the discardingof message cells. Examples of these are virtual connections for whichthe declared connection parameters with respect to the transmission bitrate are not adhered to.

FIG. 2 schematically shows the formation of one of the logical weightlists Q1-Qm according to FIG. 1. In accord therewith, such a weight listis realized by a simple, chained list of memory locations of the buffermemory PS. Given such a chaining of memory locations, the start and theend of a logical weight list Q1-Qm are marked by a start pointer(address) and an end pointer. Over and above this, an additional pointer(address) that references the following memory location of the weightlist Q1-Qm is maintained in each memory location belonging to a weightlist Q1-Qm. Only the memory location forming the end of a weight listQ1-Qm comprises no such pointer. This is indicated in FIG. 2 by thereference NIL.

FIG. 3 schematically shows the discarding of all message cells of alogical weight list Q1-Qm. For this purpose, let the chained list shownunder a) represent a logical weight list Q1-Qm to be discarded, whereasb) indicates a free list in which all free memory locations are chainedto one another, whereby start and end are again marked by start pointerand an end pointer. For the discarding, a pointer to the first memorylocation of the logical weight list Q1-Qm to be discarded is thenentered in what is the last memory location of the free list at themoment (instead of the previous NIL). The memory location previouslyforming the end of the logical weight list Q1-Qm to be discarded undera) now represents the end of the free list (indicated with NIL).

In conclusion, let it also be pointed out that the present invention hasbeen explained merely by way of example with reference to a circuitarrangement shown in FIG. 1. Such a circuit arrangement, however, canalso be modified in terms of its circuit-oriented realization. Thus, itis possible—as indicated in FIG. 1—that a number of transmission linesections (L1) are simultaneously connected to the demultiplex meansDEMUX (FIG. 1) in order to supply the demultiplex means DEMUX withmessage cells transmitted via these transmission line sections inmultiplex mode. Moreover, a number of transmission line sections L2 canbe connected via the operating means SC (FIG. 1), the message cellsstored in the buffer memory PS being capable of being distributed ontothese.

Although modifications and changes may be suggested by those of ordinaryskill in the art, it is the intention of the inventors to embody withinthe patent warranted hereon all changes and modifications as reasonablyand properly come within the scope of their contribution to the art.

What is claimed is:
 1. A method of transmitting message cells allocatedto virtual connections which have different priorities and which aretransmitted via at least one transmission line of an asynchronoustransfer mode (ATM) system, said method comprising: passing messagecells transmitted via a transmission line through a buffer memory whichis allocated to said transmission line and which has a specific messagecell capacity; receiving a message cell for a high-priority virtualconnection on said transmission line; selecting a queue allocated to alow-priority virtual connection of message cells stored in said buffermemory in response to receiving said message cell for the high-priorityvirtual connection and in response to said buffer memory reaching saidspecific message cell capacity, wherein selection of the queue is basedon a number of message cells stored in said buffer memory for saidlow-priority virtual connection; and discarding message cells in saidqueue.
 2. The method according to claim 1, wherein discarding comprisesdiscarding all message cells in the queue that are in said buffermemory.
 3. The method according to claim 1, wherein discarding comprisesdiscarding all message cells in the queue that are stored in said buffermemory except for a first of the message cells in the buffer memory. 4.The method according to claim 1, wherein selecting is performed for avirtual connection having a number of message cells stored in saidbuffer memory.
 5. The method according to claim 1, wherein selecting isperformed for a virtual connection having a greatest number of messagecells stored in said buffer memory.
 6. The method according to claim 1,wherein selecting is based on priorities of said virtual connections anda number of message cells stored for individual virtual connections. 7.Circuitry for transmitting message cells that are allocated to virtualconnections and that have different priorities via at least onetransmission line of an asynchronous transfer mode (ATM) system, thecircuitry comprising: a buffer memory having a specific message cellcapacity, the message cells passing through the buffer memory duringtransmission; a memory controller that allocates the buffer memory to atransmission line and that allocates logical queues to virtualconnections on said transmission line; wherein the memory controllerselects a logical queue allocated to a low-priority virtual connectionin response to arrival of a message cell of a high-priority virtualconnection on said transmission line and in response to said buffermemory reaching said specific message cell capacity, the memorycontroller selecting the logical queue based on a number of messagecells stored in said buffer memory for the low-priority virtualconnection, and wherein said message cells allocated to said selectedlogical queue are discarded.
 8. The circuitry according to claim 7,wherein said memory controller discards all message cells indicated in aselected logical queue.
 9. The circuitry according to claim 7, whereinsaid memory controller discards message cells indicated in a selectedlogical queue except a first of the message cells.